(1) Field of the Invention
The present invention relates to an enclosed alkaline storage cell, more particularly, to a non-sintered cadmium plate.
(2) Description of the Prior Art
A lead storage cell and a nickel-cadmium storage cell are conventionally utilized as a secondary cell. Generally, the nickel-cadmium storage cell is widely utilized in the fields covering from compact appliances to a space development because of its superior characteristics such as better discharge characteristics in a high current and a longer charge/discharge cycle compared with those of the lead storage cell. With the improvement in the performance of devices in these fields, longer reliability has been required for the nickel-cadmium storage cell.
The following are assumed to be the causes of how the nickel-cadmium storage cell comes to the end of its life.
(Cause 1) Dryout, that is, the internal electrolytic solution of the cell is released to the exterior thereof by means of a safe valve when the internal pressure becomes higher due to gas generation caused by overcharge.
(Cause 2) An internal shortcircuit with the progress of migration, that is, cadmium accumulates and fills up micro-holes of a separator through re-precipitation by a repetition of dissolution-precipitation reaction of the materials of the negative electrode in the course of charge and discharge.
The end of the cell life by dryout is caused by generation of inconsumable hydrogen gas from the negative electrode within the cell, which happens in the following conditions;
when the cell is overcharged, or
when the chargeable capacity of the negative electrode becomes smaller than that of the positive electrode during charge/discharge cycles, which results in that the negative electrode fully charged faster than the positive electrode when the cell is charged.
However, such a phenomenon hardly happens under the normal usage as it can be prevented by providing a larger chargeable capacity appropriately to the negative electrode.
Consequently, a shortcircuit is assumed to be the major cause for deteriorating charge/discharge cycles. Although a cell having a paste negative electrode has become popular with its facility in obtaining a high energy density and lower operation cost, migration is observed conspicuously therein compared with a cell having a conventional sintered negative electrode. This is caused by an organic material as a binder included in the paste for the negative electrode in order to bind an active material together, which is not included in a cell having the sintered negative electrode because an active material is included in its matrix of sintered metal therein. More specifically, migration happens because the binder is oxidized or dissolves in the course of a repetition of charge/discharge cycles and comes to not to function as the binder for fixing the active material inside of the plate. Particularly, migration is accelerated when a bulk density of the active material is heightened in order to produce a cell with a larger capacity. Because when the cell is charged and discharged, the higher the bulk density becomes, the more the volume of the active material is fluctuated.
The following are methods proposed in order to prevent the short circuit caused by migration.
1) A method for thickening the separator or making micro-holes thereof smaller.
2) The method for adding quaternary ammonium to an electrolytic solution as disclosed in Japanese Laid-Open Patent Application No. 50-91728.
3) The method for adding alkali metal salt of silicic acid to the electrolytic solution as disclosed in Japanese Laid-Open Patent Application No. 56-32744, wherein a sintered negative electrode is utilized.
4) The method for dissolving a negative electrode with a nickel solution in order to form microporous nickel layers on the surface thereof and, further, adding a magnesium compound thereto as disclosed in Japanese Laid-Open Patent Application No. 2-90461.
However problems as follows still remain even with these methods.
The first method has a problem that the thicker the separator becomes, the larger area it requires inside the cell, which results in allocating smaller area for the electrodes. Therefore, an energy density of a cell is deteriorated as the total amount of the active material is decreased in response to the smaller area for the electrodes. Moreover, as the maximum thickness of the separator is not very great in a cell for practical usage, excellent improvement of charge/discharge cycle characteristics is not realized.
When micro-holes of the separator are produced in a smaller size, gas does not easily transmit through the separator. Under these conditions, oxygen gas generated from the positive electrode when the cell is fully charged does not reach to the negative electrode to be consumed thereat. Thus, the performance of the cell is deteriorated as the internal pressure thereof becomes higher due to the unconsumed oxygen gas.
The second method has a problem that oxygen gas is not absorbed efficiently when quaternary ammonium is added, which results in inefficient performance of the cell as the internal pressure becomes higher.
The third method has a problem that satisfactory effect is not obtained with a cell having a paste negative electrode when silicic acid salt is added. Moreover, charge/discharge cycle characteristics are deteriorated because silicic acid salt turns the active material into inactive plane crystals.
The fourth method has a problem that manufacturing cost becomes higher as it requires a galvanizer and electrical equipment as well as more complicated manufacturing process.